January 2003

Slugs and Snails: Constant Nemeses of Ornamentals By Ron Oetting

Slippery! Slimy! Creepy things, yuck! This pretty muchdescribes the general opinion of what a slug or snail is. These pests fallunder the general responsibility of entomologists when it comes to who has todeal with calls and questions concerning their control, but they are far frombeing an insect. In fact, they are not even in the same phylum.

Slugs and snails belong to the phylum Mollusca, which putsthem in the same grouping as oysters, octopods, crustaceans and clams. They arein the subgroup Gastropoda, which includes animals with a head, a ventralmuscular foot attached to the abdomen and a shell. In fact, most of theseashells you collected walking on the beach during your last vacation weremembers of Gastropoda. No, slugs do not have a visible shell, though theyactually have a much-reduced internal shell. Snails have an obvious externalshell that is large enough to hold the entire body. The bodies of both groupsare soft, unsegmented and yes, slimy. The head has two pairs of tentacles: one pairis short, located on the front of the head and used for touch and smell; theother pair of tentacles is longer, located on top of the head, with an eye atthe distal end of each tentacle. Slugs and snails are hermaphrodites,possessing both male and female organs. Even though they can beself-fertilized, they usually mate with another slug or snail and may use bothmale and female organs during mating. In some situations, they may be male orfemale during part of their life, in addition to being a hermaphrodite.

There are many more species of land snails than slugs, andmost do not cause noticeable damage to ornamental plants. However, the manythat do are among the most damaging pests in the greenhouse, garden andlandscape. The slugs and snails that cause most of the damage are introducedspecies that are not native to the area. They are usually gregarious and buildup large populations in a localized area, potentially causing severe damage.Even though slugs and snails are not insects, they are very similar in theirbiology and behavior.

Many species of slugs and snails are economic pests ofornamental plants. The most noteworthy snail species is the brown garden snail(Helis aspersa), which was initially introduced into California from France inthe 1850s. It was brought into the United States to be used as food but escapedinto the environment and has been a major pest since then. There are severalspecies of snails that are common in moist areas, but most of the damage isfrom their presence on plants that are being shipped to market. Some snails arepredators and feed on other snails, while others feed primarily on algae andother vegetative material common in wet areas. In much of the Southeast, slugsare the most damaging group. Some of the important species are brown slug,garden slug, gray garden slug, greenhouse slug, Lehmannia slug and spottedgarden slug.

A slug/snail life

Slugs and snails move by sliding along on a muscular foot.Wave-like movements on the contact portion of the muscular foot on the mucuslayer secreted by a gland behind the mouth and smaller glands on the footpropel them along. The protection of the slippery mucous allows them to moveover very rough surfaces. This foot is very sensitive to particles andchemicals, and it is a common target to many of the home remedies used againstsnails and slugs. Once the animal has passed, the mucus dries leaving thetelltale shiny slime trail that is characteristic of these pests and is the keyto identifying that they are the villain that chewed holes in your plants thenight before.

Snails and slugs are most active at night, but they can beout during the day if it is overcast and especially if it is foggy or rainy. Whenthe sun is out, they seek a hiding place out of the dryness and heat. As thetemperature increases, they seek shelter in a cooler area to protect themselvesfrom desiccation during the day. They can commonly be found under boards,leaves, rocks and any other dark cool place that is protected from the sun.Some species will even burrow into the soft earth where they will rest andfeed. Eggs are laid in these protected areas, reducing the chance of predationby their natural enemies. They are active spring through fall and hibernate inprotected areas during the winter. In warmer climates and greenhouses, they canbe active all year.

The damage caused by slugs and snails is a result of chewingmouthparts. The mouthparts consist of numerous strong teethlets that allow themto remove chunks of plant material. On plants, they chew irregular holes withsmooth edges very similar to some caterpillars, making it difficult todistinguish the culprit. With slugs and snails, there is that famous slimetrail that can be used to determine which pest caused the damage. They prefersucculent plant material, so damage is most common on seedlings and herbaceousplants. They can clip seedlings off by eating through the stem and then eatingthe small leaves. Soft, ripe fruit is also attractive to slugs and snails, andthey are very damaging to strawberries, tomatoes and other soft fruit that isclose to the ground. Some species will actually burrow into soft earth and feedon the succulent roots of herbaceous plants.

Control

Control is a major problem in all habitats. There are manythings that can be done to reduce the potential of a problem occurring. A majorsource of infestation in greenhouses is the movement of plant material. Growersneed to inspect plant material as it arrives at the greenhouse for the presenceof pests that can establish themselves and become a major problem. It is a loteasier to prevent bringing in a pest than to control it once it is in thegreenhouse. Conversely, shippers need to make sure their plant material isclean before it is shipped out to growers or consumers.

If slugs and/or snails are already present, there are stillmeasures that can be taken to prevent or reduce damage. Eliminate, as much aspossible, the areas where slugs hide during the day. Anything that is sittingon the ground is a possible resting place for these slimy pests, such asboards, boxes, stones, debris, weeds, plants in pots that have runners on theground or any other items that provide shelter. Reducing hiding placesdecreases slug and snail survival. Of course, the potted plants that are set onthe ground will also provide harborage for these mollusks. Placing pots onraised benches will reduce the numbers of slugs and snails that are present onthe pots and plants. Keeping things dry will also decrease the survival andreproduction of slugs and snails and discourage them from staying in the area;consequently, they may move out of the greenhouse to find a better habitat.

Bait or traps

The reduction of hiding places decreases the numbers ofslugs and snails, but if you are going to use bait or traps for control, hidingplaces can be used as a location for baits. Slugs and snails will be attractedto these areas, increasing the probability of them eating the bait. Traps canbe constructed of boards with runners on each side to allow room for entry, orold pieces of wet carpet might be used. Traps are then checked regularly, andthe slugs are removed and destroyed. Some people recommend the use of beer,cooked cabbage, dog food, grape juice, veggie mix, etc., to attract slugs andsnails to an area. These methods have been reported with various claims ofsuccess. Some studies have even looked at different brands of beer and whetherit is best used fresh or stale. You will need to monitor and refresh baits atregular intervals. The use of a food source under a trap board should increasethe numbers of these pests collected.

Traps can also be sunken into the ground and coated withsoap or grease to trap slugs and snails. Once they enter the slippery-sided jaror plastic container, they cannot climb back out. The addition of your favoritebait should increase the catch. In landscape situations when you are trying toprotect a small area, barriers can be placed around the area to reduce movementinto it. Some of the barriers used are coffee grounds, copper strips,diatomaceous earth, horseradish roots, lava rock, lint from the dryer, nutshells, rosemary sprigs, salt, sand, sandpaper, the spiny fruit of sweet gumtrees, wood ashes and many more home remedies. These do not have muchpracticality when trying to protect a large area of plants. There are manynatural predators of slugs and snails, including insects such as groundbeetles, snakes, frogs and toads, turtles, lizards and other snails. One of themajor groups of predators is birds. Many birds feed on slugs and snails, and Ihave visited greenhouses where quail are kept inside the greenhouses to feed onslugs, snails and other ground pests.

Chemical control

Chemical control is the common method used when protectingornamental plants. There are very few pesticides registered for use againstÁ slugs and snails, so the common method of application is spreadingbaits. The key to control is the attractiveness of the bait. To obtainmortality, the bait must attract the mollusk and stimulate it to consume thebait containing the pesticide. If the bait is old, stale or rancid, it will beless likely to succeed. The most common bait on the market is metaldehyde, andthere are different formulations available. The attractiveness of the bait isthe most important factor; good bait can attract slugs and snails from a meteror farther. Metaldehyde does not kill slugs and snails by poisoning them, butkills them by stimulating the mucous-producing cells to over produce, so theydie of desiccation. If they do not consume enough bait, they will recover.

Another bait is methiocarb, marketed as Mesural. Care mustbe taken to make sure these baits are not deployed in piles where a pet wouldbe likely to consume the bait or where children could come in contact with it.The placement of baits in traps, as mentioned above, reduces the hazard of thewrong animal coming in contact with it. A fairly new bait, iron phosphate(Sluggo or Escar-Go) is less toxic to mammals, reducing the concern for humanor animal safety. It causes slugs to stop feeding and will take a longer periodof time for them to die.

Baits are most effective under moist conditions, so considerthe locations where baits are placed. The placement of baits in traps or inareas where slugs and snails are more likely to hide or feed will increase thecontrol with less bait. Sunlight will break down some of the chemicals, andsome bait breaks down faster than others. Mesural, carbamate (Sevin) and coppersulfate can be applied as a spray of the foliage or ground. These chemicalskill by poisoning the slimy pest.

Slugs and snails are very difficult to manage once they arewell-established. Most of the damage to ornamentals is a result of slugfeeding, but some snails, especially the brown garden snail, are just asdamaging. There are several university and industry Web sites that addressthese pests if more information on the biology and management of slugs andsnails is needed.